Microstrip transmission lines are widely used in microwave applications. Such transmission lines include a dielectric substrate having a ground plane on one side and a conductive line on the other side, which together form a transmission line having a characteristic impedance, typically 50 ohms. The transmission line structure is fabricated using conventional photolithographic or screen printing techniques. Microstrip transmission lines are used to interconnect components in a microwave system and are typically mounted with other components in a metal enclosure or housing.
When the microstrip transmission line on the chosen substrate is used at frequencies in the gigahertz range, the linewidth to provide a 50 ohm impedance is on the order of about 0.6 millimeter. In order to interconnect such a transmission line to an RF connector or to other microstrip circuits, it is necessary to provide a contact pad having larger dimensions than the transmission line. The contact pad has a different impedance from the remainder of the transmission line because of parasitic capacitance between the excess width of the contact pad and the ground plane. As a result, there is a mismatch in the transmission line impedance, which causes undesired reflections.
It is known in the prior art to reduce or eliminate the parasitic capacitance of a contact pad by removing the ground plane on the back of the substrate and by providing a recess in the housing adjacent to the contact pad. While this technique is effective, it increases the cost of the assembly since the ground plane must be patterned, and additional machining is required to form the recess in the housing.
An additional disadvantage of removing the ground plane is that is fails to compensate for the parasitic impedance of the connection element which is attached to the contact pad. It is therefore desirable to provide a compensation technique which compensates for the impedance of the contact pad without increasing the manufacturing cost of the assembly.
It is a general object of the present invention to provide improved microstrip transmission lines.
It is another object of the present invention to provide microstrip transmission lines having compensation for contact pad impedance.
It is a further object of the present invention to provide microstrip transmission lines which are low in cost and easy to manufacture.
It is still another object of the present invention to provide microstrip transmission lines wherein the parasitic impedance of contact pads is incorporated into a filter.